The present invention relates to a process for the continuous production of epichlorohydrin (ECH) from dichlorohydrins (DCH), with the 1,3 type prevailing over the 1,2 type, in presence of an alkali solution, wherein the DCH's are produced by a continuous catalyzed process using raw or purified glycerol and hydrogen chloride as feedstocks.
Epichlorohydrin is produced by dehydrochlorination of the DCH by means of an aqueous solution of alkali.
Despite there are different processes for the production of epichlorohydrin, such as the conventional process via allyl chloride starting from propylene and chlorine, the Showa Denko process using allyl acetate as reaction intermediate, and the process starting from glycerol and hydrogen chloride, also called GTE (glycerol to epichlorohydrin), brought to the industrial stage only recently, but having its technological principles well known since more than one century, the dehydrochlorination process in all these processes has been subject to limited improvements.
The principles of this process is to allow a dehydrochlorination reaction, in which the dichlorohydrins (1,3 or 1,2) are converted to epichlorohydrin in presence of an alkali, with the formation of water and of the corresponding chloride salt, as explained in the reaction scheme below:

The rate of conversion is high, somewhat more rapid for the 1,3 isomer in comparison with the 1,2 isomer, and it is generally favored by increasing the temperature or the alkalinity.
The yields of the dehydrochlorination reactions (1) and (2) above are limited by the presence of side reactions.
The main side reaction is the hydrolysis of the ECH to monochlorohydrin (3), glycidol (4) or glycerol (5):

The hydrolysis of epichlorohydrin is catalyzed by alkali or acids, but it takes place, even if at a lower rate, also at neutral conditions.
Direct condensation of epichlorohydrin, favored by alkaline conditions, can lead to the formation of poly-glycols. Some chloroethers may be formed by the reaction of a dichlorohydrin with epichlorohydrin.
Since all the side reactions occur in the liquid phase, the best method to increase the overall yields is to strip-out the epichlorohydrin as soon as it is formed, feeding the crude dichloropropanol, after mixing with alkali, in counter-current flow of steam in a reactive distillation type column.
Moreover, it is well known since more than 40 years by the engineers and operators skilled in the dehydrochlorination problems that a residence time not higher than 20 seconds, and even better not higher than 10 second, shall be used in the reaction trays to avoid the degradation of epichlorohydrin.
As regard the operating conditions commonly used for this reactive distillation column, it usually works at a pressure slightly higher than atmospheric, and at temperatures between 100 and 120° C. (temperature at the column bottom).
The heat to the column is usually provided using live steam, with or without a vapor thermocompression of the bottom product of the column, consisting in salted water, which is depressurized and partially vaporized under slight vacuum conditions.
The heavy by-products formed in the reactive column, as glycidol, glycerol and poly-glycerol, are not stripped by the live steam and, therefore, they are lost in the aqueous bottom stream together with the salts. As a consequence, the inefficiency in the reaction impacts also in the quality of the waste water subject to a treatment, before to be disposed or recycled.
Alkali type and activity also affect the reaction yields; the more common calcium hydroxide Ca(OH)2, as freshly prepared and homogenous water slurry, is used or, as a different option, caustic soda, as NaOH diluted solution, can also be used.
The major difference in the operating conditions of this column between the conventional process (from chlorine and propylene) and the process of the present invention (from glycerol and hydrogen chloride), is the different concentration in DCH of the feedstock. In fact, while in the conventional technology a very diluted stream of DCH's in water (around 3-4% wt) is fed to the dehydrochlorination, in the process according to the invention, the DCH's are produced in a very high concentration (usually more than 95% wt). This feature of the new process allows to produce a much lower amount of water effluent from the bottom of the dehydrochlorination column, but, on the other side, it requires the use of caustic soda instead of milk of lime slurry, due to the severe trays fouling and the operating problems which can occur. Unfortunately the use of caustic can be the cause of further problems because, owing to its reactivity higher than milk of lime, it may produce an higher quantity of by-products due to the faster re-conversion of the produced ECH to glycidol or glycerol.
Another important difference of the process from glycerol according to the invention is the nature of the dichlorohydrins. While in the conventional process the reaction between allyl chloride and hypochlorous acid produces mainly the 1,2 DCH compound, in the new process the catalytic reaction of glycerol with hydrogen chloride produces 1,3 DCH with a selectivity higher than 90%. It is known from the technical literature than the 1,3 dichlorohydrin is much more reactive than the 1,2 DCH, because the intermediate ion bringing to the formation of epichlorohydrin of the 1,2 compound is less stable.
A third point of distinction of the process from glycerol according to the invention, is connected to the production of water in the hydrochlorination reaction of the glycerol with hydrogen chloride and the consequent advisability to remove at least part of the reaction water to favor the reaction kinetic and conversion. The removal of water by evaporation is necessarily joined by a partial vaporization of hydrogen chloride, which has to be neutralized or recovered as hydrochloric acid solution, and of dichlorohydrins which shall be recovered through their conversion to epichlorohydrin.
In conclusion the higher reactivity of both the reactants of the process of the present invention, the 1,3 DCH and the sodium hydroxide, compared to the 1,2 DCH and the calcium hydroxide respectively mostly present in the conventional process, together with the much higher concentration of the dichlorohydrin stream and to the possible presence of a second vapor feed, suggest the use of a different arrangement and different operating conditions of the reactive system to produce epichlorohydrin.
Different patents have been published related in general to the process of producing ECH, and some particularly related to possible solution of one or more of the problems listed above. Here below are reported the most significant.
U.S. Pat. No. 2,177,419 published on Oct. 24, 1939 may be considered among the first applications considering the reaction of the dichlorohydrin, produced from allyl chloride, with an excess of basic metal hydroxide, particularly with an aqueous slurry of calcium hydroxide, and the following stripping of epichlorohydrin with live steam at atmospheric pressure. The patent also remarks the importance of the low residence time in the reactor. Among other things, it quotes as follows: if the contact time of the epihalohydrin with the alkaline reaction mixture is made negligible at the temperature of separation of the former therefrom, higher separation temperatures may be used without substantial destruction of the epihalohydrin.
British patent GB 799,567 and European patent EP 0421379 try to solve this problem using a solvent extraction (trichloropropane or glycol-ether respectively), to separate the produced epichlorohydrin from the aqueous phase.
The Japanese patent JP 52000210 relates to a saponification column using perforated trays without liquid downcomers to minimize the residence time, and another Japanese patent JP 59196880 describes a saponification column with combined feed of milk of lime and caustic soda solution, to avoid loss of product in the column bottom.
U.S. Pat. No. 4,496,53 relates to a double step saponification process, made by a back-mix reactor and a plug-flow reactor, both in presence of an inert organic solvent, as carbon tetrachloride.
The Japanese application JP 3145481 solves the problem of unconverted DCH loss in the top of the column, by using, before the total condenser, a partial condenser to recover, by refluxing them to the column, the DCH lost in the top with the product.
The subject of the Japanese patent JP 63017874 is a saponification column, where DCH and milk of lime are not premixed before entering in the column, but where the milk of lime feed is fed separately at a higher position respect to the DCH feed.
The Czech application CZ 293728 describes a saponification column designed in a way that the tray liquid load, in order to minimize the residence time, is the minimum possible to maintain hydraulic stability.
More recent patent applications related to the saponification column in a epichlorohydrin from glycerol process are between the others: WO 2011/092270, where membranes are used in the saponification column to separate the two phases of the distillate, JP 2009/263338, where the ratio 1,3/1,2 DCH is fixed to control the properties of the organic and the aqueous phases in the saponification distillate, and JP 2009/184943, where, instead of a reactive column, a CSTR type reactor is used with a simultaneous distillation of the produced epichlorohydrin.
Other patents from Solvay S.A. relate to the dehydrochlorination step of the process to produce epichlorohydrin from glycerol. The invention of EP 2132190 and EP 2160356 describes a process for producing epichlorohydrin, wherein the glycerol dichlorohydrin reacts with a basic compound and the product of reaction is subjected to a decantation process, where one fraction contains most of the epichlorohydrin produced and a second aqueous fraction contains most of the salt produced in the reaction. Substantially it represents a method rather different from the conventional reactive column.
Other three patents by Dow Global Technologies US 2010/0029960, U.S. Pat. No. 7,982,061 and U.S. Pat. No. 7,985,867 relate to a process and apparatus for the dehydrochlorination of dichlorohydrins with production of epichlorohydrin. The first, more specifically, refers to the mixing of the dichlorohydrin stream with a base water solution in a plug flow mixer-reactor, the second to the operating conditions of the reflux drum to be used to minimize the hydrolysis losses, the third wherein the inventive step lies in maintaining the liquid holdup per tray below certain values of residence time, depending from the section of the column (top, feed and bottom zones). Each of the three patents refers to a particular feature able to reduce the hydrolysis reactions with by-products formation. All the examples included in these patents refer to a stream of dichlorohydrins produced by glycerol chlorohydrination.
As shown in the above patents description, during the years have been done some efforts to solve separately the problems related to the saponification reaction, but none of the patents is related, in a complete and organic way, to the dehydrochlorination column for solving the different problems and, at the same time, this problem has not yet been considered taking into account the new features of the processes for production of epichlorohydrin from glycerol. The scope of this patent is to propose a dedicated process for the dehydrochlorination, that allows to improve the performances of the column in terms of higher yields in epichlorohydrin and better quality of the waste water produced at the column bottom.